Suppose we have an object
And object has an initial velocity
Let us decide the direction be anywhere
If then that object is allowed to travel
Freely but under gravity.
The same motion is called projectile motion

In case of projectile motion,
Accelaration due to gravity is constant
The effect of air resistance is negligible

It is the two dimensional motion
Can be regarded as simultaneous superposition of two motions.
One horizontal with velocity "u"cos theta and accelaration=0.
Other verticle with velocity "u" sin theta and accelaration=-g.